7O ALASKA GEOLOGY 



without any pretense to accuracy, intended only to show 

 the general relative distribution of the various formations 

 discovered. 



VICINITY OF SAND POINT, POPOF ISLAND 



Dall in his report on Coal and Lignite of Alaska 1 states 

 that the northwestern part of Popof Island, near Sand 

 Point, is composed of sandstones and conglomerates sim- 

 ilar to those of the Kenai Formation in Coal Bay on the 

 neighboring Unga Island. " They are broken and cut by 

 dikes and larger intrusions of basaltic lava and diorite and 

 near the contacts are much altered and intersected by 

 veins of chalcedonic quartz." 



I did not see this sedimentary formation. The point 

 south of the harbor at Sand Point was visited, and seemed 

 to be made up wholly of lavas and volcanic tuff; and 

 similar rocks were found on the northern shore of the 

 island at a bluff about three miles from Sand Point. 



The lavas at Sand Point were found to be augite-an- 

 desite and augite-hypersthene-andesite. They are dark 

 grey compact rocks showing glassy feldspar crystals in 

 a dull groundmass. Under the microscope the struc- 

 ture is strongly porphyritic (slides 125, 126), the most 

 abundant crystals being large fresh plagioclase feldspars 

 with extinction angles of labradorite, abundantly twinned 

 but very slightly zoned, and full of inclusions of glass. 

 Augite, colorless or pinkish, is also present, but is largely 

 altered to green serpentine. One slide (127) shows abun- 

 dant hypersthene, sharply idiomorphic, with characteristic 

 double refraction and faint pleochroism; some hypersthene 

 crystals are enclosed in augite crystals in parallel position. 

 The groundmass contains microlites of feldspar and augite 

 in a glassy base. 



Beneath the heavy masses of these very fresh lavas, 



1 17th Ann. Rep. U. S. Geol. Survey, pt. i, p. 808. 1896. 



